Photocatalytic Depolymerization of Native Lignin toward Chemically Recyclable Polymer Networks

As an inedible component of biomass, lignin features rich functional groups that are desired for chemical syntheses. How to effectively depolymerize lignin without compromising the more valuable cellulose and hemicellulose has been a significant challenge. Existing biomass processing procedures either induce extensive condensation in lignin that greatly hinders its chemical utilization or focus on fully depolymerizing lignin to produce monomers that are difficult to separate for subsequent chemical synthesis. Here, we report a new approach to selective partial depolymerization, which produces oligomers that can be readily converted to chemically recyclable polymer networks. The process takes advantage of the high selectivity of photocatalytic activation of the β-O-4 bond in lignin by tetrabutylammonium decatungstate (TBADT). The availability of exogenous electron mediators or scavengers promotes cleavage or oxidation of this bond, respectively, enabling high degrees of control over the depolymerization and the density of a key functional group, C=O, in the products. The resulting oligomers can then be readily utilized for the synthesis of polymer networks through reactions between C=O and branched −NH2 as a dynamic covalent cross-linker. Importantly, the resulting polymer network can be recycled to enable a circular economy of materials directly derived from biomass.


Klason Lignin Information S5
Carbonyl Quantification Experiments S5 Table S1 Summary of PPol photocatalysis under varying conditions S13 Table S2 Summary of PPol vs. PPdiol photocatalysis products S13 Table S3 Summary of native lignin photocatalysis results under varying conditions S14   Figure S4: SEC of catalysts, crude oligomers, and purified oligomers.

Klason Lignin Information
Klason lignin content was measured according to a literature procedure in order to calculate the yield of lignin oligomers. 1 In a typical experiment, woodmeal (0.2 g) and 72% sulfuric acid (1.5 mL) were mixed in a glass vial, and stirred for 1 hour at 30 °C in a water bath. The mixture was diluted to 42 mL with DI water and transferred to an autoclave reactor. The solution was heated to 121 °C for 1 hour. After cooling to room temperature, the brown precipitate (Klason lignin) was separated by filtration. The solid was washed with DI water (3 x 20 mL). The Klason lignin was dried at 105 °C for 24 hours to afford the product (16.1 ± 0.1%).

Carbonyl Quantification Experiments
To calculate the amount of carbonyls in the lignin oligomers, a published method about using 4-(trifluoromethyl)phenylhydrazine to derivatize carbonyls to the corresponding hydrazone was applied. 2 For a typical quantification reaction, lignin oligomers (15 mg), 100 μL p-trifluoromethyl toluene (Internal standard, IS) in DMSO-d6 solution (0.1682 μmol/mg, accurately weighed), 200 μL 4-(trifluoromethyl)phenylhydrazine in DMSO-d6 solution (containing 70.5 mg 4-(trifluoromethyl)phenylhydrazine, 0.40 mmol), and 400 μL pure DMSO-d6 were mixed in a 4 mL vial, and then homogenized in a vortex mixer. The mixture was then transferred to a NMR tube, and heated to 45 °C for 24 hours. Before NMR analysis,100 μL of relaxation agent solution in DMSO-d6 (containing 2.8 mg chromium acetylacetonate) was added and homogenized. 19 F NMR was carried out with the following acquisition parameters: 90° pulse angle, relaxation delay = 3 sec, scans = 64, region = -50-70 ppm. IS was used as a reference peak set to -60.9 ppm, with the unreacted hydrazine peak at -59 ppm. Figure S5: NMR of the hydrzaone formation for the quantification of carbonyl concentration in DMSO-d6 600 MHz.

S6
The total amount of carbonyls present is calculated as follows:  Figure S6: Comparison of the yields of the photocatalysis of native lignin to lignin oligomers under varying atmospheres.

Safety Statement
No unexpected or unusually high safety hazards were encountered.

Materials
All reactions were carried out under a nitrogen atmosphere unless otherwise stated. Nitrogen and oxygen gasses were purchased from airgas. Beech wood sawdust was purchased from Blegwood. All solvents including ethanol, acetone (AC), anhydrous acetonitrile (AN), dichloromethane (DCM), dioxane, ethyl acetate (EA), hexane (Hex), methanol (MeOH), and tetrahydrofuran (THF) were purchased from commercial sources and used without further purification unless otherwise stated. Dioxane was further dried with a drying column of a solvent system from Pure Process Technologies. NMR solvents including dimethyl sulfoxide (DMSO-d6) and chloroform (CDCl3) were purchased from Cambridge Isotope Laboratories or ACROS organics. Sodium tungstate dihydrate and tetrabutylammonium bromide (TBAB) were purchased from Acros organics. Diphenyl acthracene (DPA) was purchased from Alfa Aesar.

Light Reactor Specifications and Setup
The light source was a 200 W UV LED light centered at 365 nm, from Howsuper, with the source number H6015-S-6868-LG-365nm.
Figure S11: Light setup used for photocatalysis.

TBADT Catalyst Synthesis
TBADT catalyst was synthesized according to a literature procedure with minor modifications. 3 TBAB (4.8 g, 15 mmol) and Na2WO4·2H2O (10.0 g, 30 mmol) were dissolved in separate deionized (DI) water (50 mL and 100 mL, respectively) solutions. The solutions were acidified to pH 2 with concentrated hydrochloric acid. After heating to 90 °C, the solutions were mixed together. Precipitation was observed immediately, indicating the formation of TBADT. The slurry was stirred for 30 minutes in a 90 °C water bath, cooled to room temperature, and filtered with a Büchner funnel. The solid phase was washed with DI water and THF (3 x 30 mL), and dried in a vacuum oven at 90 °C overnight. The crude TBADT was further purified by recrystallization. The crude TBADT was refluxed in DCM (1 g: 20 mL) for 2 hours. The mixture was cooled on an ice bath, and then filtered to obtain pure TBADT as a transparent crystal with a light yellow color (4.02 g, 40.0% based on W).

Photocatalysis of PPol or PPdiol without DPA
For a typical photocatalytic reaction of PPol or PPdiol without DPA, PPol (10.7 mg, 50 μmol) or PPdiol (12.2 mg, 50 μmol), TBADT (5.3 mg, 1.6 μmol), and AN (1 mL) were added to a 10 mL Schlenk tube. The solution was degassed by the freeze-pump-thaw method (3 x) such that the atmosphere of the schlenk tube was 1 atm nitrogen. The solution was then heated to 50 °C in a water bath and irradiated with UV light for 2 hours. The products were then analyzed and quantified by 1 H NMR (48.0% conversion, 9.1% PPone, 29.2% acetophenone, 9.7% benzaldehyde). See Table S1 for yield details.

Photocatalysis of PPol with DPA
The procedure is the same as the photocatalysis of PPol without DPA. DPA amounts used were as follows: 66  Table S1 for yield details.

Photocatalysis of PPol with Oxygen:Nitrogen Mixture Atmosphere
Varying oxygen mixtures were prepared by mixing pure oxygen and pure nitrogen in a high-pressure reactor. For 2%, 5% and 20% oxygen, the partial pressures for oxygen and nitrogen were 0.5 bar/24.5bar, 0.5 bar/9.5bar, 2 bar/8 bar, respectively. The procedure is the same as the photocatalysis of PPol reaction without DPA; however, the oxygen:nitrogen gas mixtures were introduced instead of pure nitrogen during freeze-pump-thaw cycles. Oxygen percentages used are as follows: 2% oxygen or 5% oxygen or 20% oxygen See Table S1 for yield details.

Photocatalysis of Native Lignin without DPA
For a typical photocatalytic reaction of native lignin, woodmeal (2.50 g), TBADT (250 mg, 75 μmol), and AC:AN (30 mL 2:1) were mixed together in a 3 oz pressure reaction vessel. The system was degassed by freeze-pump-thaw in the same manner as the photocatalysis of PPol. The reactor was heated in a water bath to 50 °C and stirred for 48 hours under UV light irradiation. The oligomer solution was then collected by filtering the resulting slurry. The solution was concentrated under reduced pressure and the residue was resuspended in THF and sonicated for 5 minutes. The solution was then passed through a syringe filter (PET-45/25 polyester membrane with pore diameter of 0.45 µm, and membrane diameter of 25 mm) to remove the precipitated TBADT. The crude oligomers were purified with silica gel column chromatography. A mixture of hexane and ethyl acetate (8:2) was used to remove non-polar impurities. The gradient was then switched to DCM and methanol (1:0-0:1) to obtain the remaining oligomers. The collected oligomers were then subjected to a neutral aluminum oxide plug (DCM:MeOH 1:0-0:1) to further purify the oligomers (58.2 mg, 14.5%). See Table S2 for yield details.

Photocatalysis of Native Lignin with DPA
The procedure is the same as the photocatalysis of native lignin without DPA. DPA amounts used were as follows:  Table S2 for yield details.

Photocatalysis of Native Lignin with Different Atmospheres
The procedure is the same as the photocatalysis of native lignin without DPA; however, either oxygen gas or air (1 atm) were introduced instead of pure nitrogen during freeze-pump-thaw cycles. See

Preparation of Organosolv Lignin
Organosolv lignin for 2D-HSQC NMR was synthesized according to the literature. 4 Pretreated woodmeal (5 g), 80% ethanol aqueous solution (40 mL), and concentrated hydrochloric acid (0.8 mL) were added to a 100 mL round-bottom flask. The mixture was refluxed at 80 °C for 5 hours at which point the mixture was filtered. The residue was washed with 100% ethanol (4 x 5 mL). The filtrate was collected and concentrated under reduced pressure. The remaining solid from the filtrate was re-dissolved in acetone (5 mL). The solution was then precipitated into DI water by dropwise addition (100 mL). Upon addition to the water, a brown-pink precipitate formed. The solution was filtered and the collected solid was dried in a vacuum oven at 45 °C overnight to afford the product (0.25 g, 5%).

S15
Beech wood sawdust was pretreated according to a literature procedure with minor modifications. 5 The wood sawdust (1 g:10 mL) was suspended in an ethanol-water solution (4:1) and sonicated for 2 hours to remove most of the extractives. The solution was filtered, and the solid was washed with acetone (3 x 100 mL), and dried in a vacuum oven at 45 °C for a minimum of 48 hours. The solid was then oven-dried in 2 hour intervals at 105 °C until the weight change was <5% indicating a <5% moisture content. The dry sawdust was then knife-milled through a 1 mm screen. The wood powder was sealed and stored at 0 °C.

Control Experiments with Cellulose and Hemicellulose
The photocatalysis on cellulose and hemicellulose featured the same conditions and additives as typical woodmeal photocatalysis above, with the only difference being the reactant: using pure cellulose or hemicellulose (1.00 g) instead of woodmeal (2.50 g). After the reaction, the solution was filtered and washed with AN (3 x 5 mL), to remove TBADT, dried in vacuum oven overnight, and weighed to calculate the remaining percentage (weight after reaction ÷ weight before reaction) of each species. After photocatalysis, cellulose (95.4%) and hemicellulose (82.7%) were recovered. See Figure S9.

Synthesis of Polymer Filler
MMA (2.00 mL, 18.76 mmol) and AAEMA (358.14 μL, 1.88 mmol) were added to a flask and dissolved in dioxane (15 mL). AIBN (4.62 mg, 28.14 μmol) was added as a dioxane (1 mL) solution. The system was sealed with a septum and subjected to 3 rounds of freeze-pump-thaw to remove oxygen. The solution was heated to 70 °C for 29 hours at which point the solution was concentrated by half and precipitated into hexane (3 x 45 mL). The product was obtained as a yellow solid (2.00 g). The NMR spectrum is consistent with the literature 6 and is shown below ( Figure S12) for the calculation of AAEMA incorporation.

S16
As an example, lignin (34 mg, f = 2.892 mmol/g), and copolymer (52 mg, Mn = 17.6 kDA) were dissolved in THF (0.296 mL) in a 1.5 mL vial. From a stock solution of TAEA (10 µL in 990 µL of THF) was transferred (6.78 µL, 4.53 x 10 -4 mmol) to the lignin solution. The solution was mixed in a circular motion and transferred within 10 seconds to a PTFE mold via pipette, in order to avoid cross-linking in the mixing vessel. Any air bubbles were carefully removed with the tip of a needle. The solution was allowed to sit at ambient conditions and undergo solvent evaporation for 18 hours at which point the film was carefully removed from the mold. The sample was cured under vacuum at room temperature for 24 hours to remove residual solvents.

Jeffamine Crosslinking Procedure
As an example, lignin (34 mg, f = 2.800 mmol/g), and copolymer (52 mg, Mn = 17.6 kDA) were dissolved in THF (0.250 mL) in a 1.5 mL vial. From a stock solution of Jeffamine (200 µL in 300 µL of THF) was transferred (44.89 µL, 4.0 x 10 -2 mmol) to the lignin solution. The solution was mixed in a circular motion and transferred within 10 seconds to a PTFE mold via pipette, in order to avoid cross-linking in the mixing vessel. Any air bubbles were carefully removed with the tip of a needle. The solution was allowed to sit at ambient conditions and undergo solvent evaporation for 18 hours at which point the film was carefully removed from the mold. The sample was cured under vacuum at room temperature for 24 hours to remove residual solvents.

TAEA Recycling Procedure
The obtained film was suspended in n-butylamine (4 mL) and heated to 80 °C until all solid was dissolved (4-18 hours). The solution was concentrated under reduced pressure. The residue was then subjected to the initial crosslinking conditions.

Jeffamine Recycling Procedure
The obtained film was suspended in n-butylamine (4 mL) and heated to 80 °C until all solid was dissolved (4-18 hours). The solution was partially concentrated to remove about half of the n-butylamine. DCM and water (20 mL) were added. The aqueous phase was extracted with DCM (3 x 20 mL). The combined organic phases were washed with water (5 x 100 mL). The combined organic phases were dried over anhydrous sodium sulfate, filtered, and concentrated under reduced pressure. The residue was then subjected to the initial crosslinking conditions.